The invention is directed to a method of digital filtering and a digital filter for processing signals with substantially differing signal levels, in particular for use in radio and radar receivers.
For a brief general discussion of digital filters, see "Year Book of Science Technology", McGraw-Hill, Vol. 1971, pages 151-153.
A special property of digital filters is that on account of the quantizing- and error rounding-off, the blocking attenuation is very dependent upon the signal level. If, due to a very low input level, a digital filter is not fully loaded, the registers of the calculating unit will not be fully exploited and as a result of the thus lower relative calculating accuracy (error rounding-off), the blocking attenuation will descend. Also, there is a specific limitation of the dynamic range. The smallest signal to be processed is precisely indicated by the lowest value bit and the highest signal is precisely indicated by the maximum permissable word length.
The objective of the invention is to eliminate these difficulties in the processing of signals over a large dynamic range by means of digital filters, said difficulties being particularly manifest mainly in radio transmission paths and, especially, in radar devices. In accordance with the invention this is achieved in that at least one signal detector is provided which establishes the level of the relevant signal at an appropriate point, and a comparison effected of the actual signal level with the maximum signal level to be processed (i.e. without limiting effect), and from these two values a ratio value is derived which, at least in the case of lower signal levels, is utilized to alter the coefficient of at least one part of the multiplication stages of the filter structure in such manner that at least a part of the individual units thereof is loaded to as full an extent as possible.
As a result, it is possible to at least partially reduce or eliminate the change in the blocking attenuation in dependence upon the signal level which would otherwise be considered as an unfavorable property of digital filters. The invention is of particular importance in all cases in which a large difference in level of the signals to be processed is to be expected from the start. This is in particular the case in radio receivers and, especially in radar receivers in which the signal levels can vary within very wide ranges (large dynamic range).
At the output of the digital filter it is expedient to provide a correction circuit, preferably in the form of an amplifier which is controllable in its amplification, which corrects the signals in accordance with the reciprocal ratio value, so that the correct signal level is obtained. This is of particular significance when the signal processing is to be continued in analogue or level-dependent fashion, for example, in the evaluation of radar echo signals when it is necessary to differentiate, e.g., between large and small targets.
An improvement along the lines of the invention is achieved even if a full loading is obtained at least in the case of part of the units of the digital filter. This is applicable, in particular, to the input stages of the filter, where error roundoff becomes especially manifest in the case of small signal levels.